22970: _new_matrix/from_columns/add_to_entry

src/sage/matrix/matrix0.pyx

@@ -508,6 +508,31 @@ cdef class Matrix(sage.structure.element.Matrix):
         """
         raise NotImplementedError("this must be defined in the derived type.")
 
+    def add_to_entry(self, Py_ssize_t i, Py_ssize_t j, elt):
+        r"""
+        Add ``elt`` to the entry at position ``(i, j)``.
+
+        EXAMPLES::
+
+            sage: m = matrix(QQ['x,y'], 2, 2)
+            sage: m.add_to_entry(0, 1, 2)
+            sage: m
+            [0 2]
+            [0 0]
+        """
+        elt = self.base_ring()(elt)
+        if i < 0:
+            i += self._nrows
+        if i < 0 or i >= self._nrows:
+            raise IndexError("row index out of range")
+        if j < 0:
+            j += self._ncols
+        if j < 0 or j >= self._ncols:
+            raise IndexError("column index out of range")
+
+        self.set_unsafe(i, j, elt + self.get_unsafe(i, j))
+
+
 ##     def _get_very_unsafe(self, i, j):
 ##         r"""
 ##         Entry access, but potentially fast since it might be without

src/sage/matrix/matrix_rational_dense.pxd

@@ -15,6 +15,8 @@ cdef class Matrix_rational_dense(Matrix_dense):
     cdef _add_ui_unsafe_assuming_int(self, Py_ssize_t i, Py_ssize_t j, unsigned long int n)
     cdef _sub_ui_unsafe_assuming_int(self, Py_ssize_t i, Py_ssize_t j, unsigned long int n)
 
+    cdef inline Matrix_rational_dense _new_matrix(self, Py_ssize_t nrows, Py_ssize_t ncols)
+
 cdef class MatrixWindow:
     cdef Matrix_rational_dense _matrix
     cdef int _row, _col, _nrows, _ncols

src/sage/matrix/matrix_rational_dense.pyx

@@ -143,6 +143,14 @@ cdef class Matrix_rational_dense(Matrix_dense):
         fmpq_mat_init(self._matrix, self._nrows, self._ncols)
         sig_off()
 
+    cdef inline Matrix_rational_dense _new_matrix(self, Py_ssize_t nrows, Py_ssize_t ncols):
+        if nrows == self._nrows and ncols == self._ncols:
+            parent = self._parent
+        else:
+            parent = self.matrix_space(nrows, ncols)
+
+        return Matrix_rational_dense.__new__(Matrix_rational_dense, parent, None, None, None)
+
     def  __dealloc__(self):
         sig_on()
         fmpq_mat_clear(self._matrix)
@@ -203,6 +211,70 @@ cdef class Matrix_rational_dense(Matrix_dense):
                 for i in range(self._nrows):
                     fmpq_set_mpq(fmpq_mat_entry(self._matrix, i, i), z.value)
 
+    def matrix_from_columns(self, columns):
+        """
+        Return the matrix constructed from self using columns with indices
+        in the columns list.
+
+        EXAMPLES::
+
+            sage: A = matrix(QQ, 3, range(9))
+            sage: A
+            [0 1 2]
+            [3 4 5]
+            [6 7 8]
+            sage: A.matrix_from_columns([2,1])
+            [2 1]
+            [5 4]
+            [8 7]
+            sage: A.matrix_from_columns((2,1,0,2))
+            [2 1 0 2]
+            [5 4 3 5]
+            [8 7 6 8]
+        """
+        cdef Matrix_rational_dense A
+        cdef Py_ssize_t ncols, k, r, col
+
+        A = self._new_matrix(self._nrows, len(columns))
+        k = 0
+        for col in columns:
+            if col < 0 or col >= self._ncols:
+                raise IndexError("column out of range")
+            for r in range(self._nrows):
+                fmpq_set(fmpq_mat_entry(A._matrix, r, k), fmpq_mat_entry(self._matrix, r, col))
+            k = k + 1
+        return A
+
+    def add_to_entry(self, Py_ssize_t i, Py_ssize_t j, elt):
+        r"""
+        Add ``elt`` to the entry at position ``(i,j)``
+
+        EXAMPLES::
+
+            sage: m = matrix(QQ, 2, 2)
+            sage: m.add_to_entry(0, 0, -1/3)
+            sage: m
+            [-1/3    0]
+            [   0    0]
+        """
+        if not isinstance(elt, Rational):
+            elt = Rational(elt)
+        if i < 0:
+            i += self._nrows
+        if i < 0 or i >= self._nrows:
+            raise IndexError("row index out of range")
+        if j < 0:
+            j += self._ncols
+        if j < 0 or j >= self._ncols:
+            raise IndexError("column index out of range")
+        cdef fmpq_t tmp
+        fmpq_init(tmp)
+        fmpq_set_mpq(tmp, (<Rational>elt).value)
+        fmpq_add(fmpq_mat_entry(self._matrix, i, j),
+                 fmpq_mat_entry(self._matrix, i, j),
+                 tmp)
+        fmpq_clear(tmp)
+
     cdef set_unsafe(self, Py_ssize_t i, Py_ssize_t j, value):
         fmpq_set_mpq(fmpq_mat_entry(self._matrix, i, j), (<Rational> value).value)
 

src/sage/matrix/matrix_rational_sparse.pyx

@@ -171,6 +171,42 @@ cdef class Matrix_rational_sparse(Matrix_sparse):
     def __hash__(self):
         return self._hash()
 
+    def add_to_entry(self, Py_ssize_t i, Py_ssize_t j, elt):
+        r"""
+        Add ``elt`` to the entry at position ``(i, j)``.
+
+        EXAMPLES::
+
+            sage: m = matrix(QQ, 2, 2, sparse=True)
+            sage: m.add_to_entry(0, 0, -1/3)
+            sage: m
+            [-1/3    0]
+            [   0    0]
+            sage: m.add_to_entry(0, 0, 1/3)
+            sage: m
+            [0 0]
+            [0 0]
+            sage: m.nonzero_positions()
+            []
+        """
+        if not isinstance(elt, Rational):
+            elt = Rational(elt)
+        if i < 0:
+            i += self._nrows
+        if i < 0 or i >= self._nrows:
+            raise IndexError("row index out of range")
+        if j < 0:
+            j += self._ncols
+        if j < 0 or j >= self._ncols:
+            raise IndexError("column index out of range")
+
+        cdef mpq_t z
+        mpq_init(z)
+        mpq_vector_get_entry(z, &self._matrix[i], j)
+        mpq_add(z, z, (<Rational>elt).value)
+        mpq_vector_set_entry(&self._matrix[i], j, z)
+        mpq_clear(z)
+
 
     ########################################################################
     # LEVEL 2 functionality